The goal of this project is to further our understanding of the pathogenesis of the autoimmune state. Two major areas have been chosen for study: 1) the role of regulatory T cells in preventing autoimmunity and 2) the role of cytokines in mediating autoimmune tissue damage. Organ-specific autoimmune disease develops in certain strains of mice following thymectomy on day 3 of life, but can be completely prevented by reconstitution of the thymectomized animal with normal adult CD4+ T cells by day 14 of life. It is therefore likely that regulatory T cells whose development is prevented by thymectomy are responsible for preventing autoimmunity throughout the lifespan of the animal. We have identified the target antigen for CD4+ T effector cells in BALB/c mice that develop autoimmune gastritis post-thymectomy as the H/K ATPase of the gastric parietal cell. Cloned lines of H/K ATPase specific T cells can readily transfer severe autoimmune disease to immunodeficient, but not normal animals. The resistance of normal animals to disease induction was shown to be secondary to the presence of a subpopulation of T cells which coexpress CD4+ and CD25+. These immunoregulatory T cells were shown to be members of a unique lineage of cells and not merely a subpopulation of T cells which had been activated in vivo in response to environmental antigens. The CD4+CD25+ cells were potent suppressors of the activation of normal CD4+CD25- cells in vitro. Suppressor activity required activation of the CD4+CD25+ population, cell contact between suppressor and effector, and was not cytokine mediated. Further studies are needed to identify the physiologic ligand(s) and mechanism of action of this unique cell population. The role of cytokines in the induction and effector function of autoreactive T cells was examined by studying the immune response of B10.S mice to immunization with myelin basic protein (MBP). Although B10.S mice should develop experimental allergic encephalomyelitis (EAE) following immunization, they are completely resistant to the development of disease and manifest an antigen-specific defect in interferon-gamma (IFNgamma) production. However, antigen-specific induction of IFNgamma and encephalitogenicity can be induced by exposure to IL-12 in vitro. This study suggested that the association between intercurrent infections and presentation or relapse of autoimmune diseases may be secondary to IL-12 production in response to pathogens. Exposure of MBP-specific B10.S T cells to bacterial lipopolysaccharide, bacterial DNA, or to an oligonucleotide containing a single CpG motif induced IL-12 production, antigen-specific IFNgamma production, and the capacity to transfer disease to naive recipients. Antibody blocking studies demonstrated that all of these activities were secondary to the capacity of these bacterial products to induce IL-12 production. The critical role of IL-12 in the induction of inflammatory autoimmune diseases was underscored by our inability to induce EAE in IL-12 deficient mice and by the demonstration that severe EAE could be induced in IFNg deficient mice by an IL-12 dependent pathway. Collectively, these results suggest that certain infectious agents may induce or potentiate autoimmune disease in genetically susceptible individuals by induction of IL-12 production.